FT HE Importance 
m= - of Chemistry 
inthe Motion Picture 
Industry. 


‘GLENN E, MATTHEWS 


ss Research Laboratory 
Eastman Kodak Company 
eae, Rochester, Nj zy, 


The Importance of Chemistry in the 
Motion Picture Industry 


BY 


GLENN E. MATTHEWS 


Research Laboratory of the Eastman Kodak Company 


in the development of photography is 
aptly illustrated by a comparison of 
the crude, cumbersome apparatus which was 
necessary to make pictures years ago with 
the simple, compact, cameras and the stable 
and highly sensitive films available today. 
When a wet plate photographer started out 
for a day’s picture-making with his pack of 
chemicals, plates, and dark tent on his back 
he resembled a prospector more than a cam- 
era man (Figure 1). Of necessity he was 
his own manufacturing plant. He chemi- 
cally sensitized his own plates just before 
using, exposed them while wet, and de- 
veloped them at once. Prints were made 
later on paper which he sensitized himself. 
With the introduction of the dry plate 
and later the film, the crude methods of 
wet plate photography disappeared, the prep- 
aration of the materials became a commer- 
cial operation, and photographers now pur- 
chase almost all the materials that they use 
from firms who manufacture them in large 
quantities. This centralization has resulted 
in a far greater improvement in quality than 
would ever have been possible by individual 
effort. 


Motion picture film was first sold in Amer- 
ica in 1889 when George Eastman supplied 
narrow film “ribbon” to Thomas Fdison. As 
now manufactured, it consists of a trans- 
parent, flexible base or support on which 
is coated a very thin layer of gelatin in 
which are suspended microscopic particles 
of a light sensitive silver salt. This upper 
sensitive layer is called the emulsion. To 
turn out millions of feet of film a year 
maintaining an unvarying uniformity of 
thickness, sensitiveness and quality requires 
a highly skilled organization backed by 
trained chemical research. In view of these 
conditions it would be quite impossible for 
an individual to prepare his own motion 
picture film. 

Experimentation must also be always in 
progress to improve the film and to find new 
methods of manufacture. In all this work, 
chemistry plays an important part, not only 


Ti PART that chemistry has played 


Figure 1—A Photographic Pioneer 


in the manufacture and treatment but later 
in the processing, after treatment, tinting, 
toning, and renovating of the film. On the 
care with which these chemical operations 
are conducted depends the wearing quality 
or life of the film. 


Chemistry in the Manufacture of 
Motion Picture Film 


In the manufacture of motion picture films 
and other sensitized photographic materials, 
absolute cleanliness is very necessary at 
every stage of the process. All operations 
must ‘be conducted in dust-free rooms and 


only clean, chemical substances: are 
used. 

Eastman motion picture film is manufac- 
tured at Kodak Park, at Rochester, New 
York. The plant consists of about 230 acres 
situated in the northwest section of the 
city. The output of this plant is roughly, 
150,000 miles of film per year. To make this 
quantity, over five million pounds of cotton 
are used yearly, several millions of pounds of 
gelatin, and over twelve tons of solid silver 


per month. 


pure, 


The water necessary to take care of the 
needs in manufacture is pumped through a 
private pipe line into large reservoirs from 
Lake Ontario, 4 miles away. The reser- 
voirs have constantly on hand 
water to supply a city of 150,000 people. The 
temperature of the workrooms is rigidly con- 
trolled at all times by refrigerating machin- 
ry, having a cooling capacity equivalent to 
the melting of 4,000 tons of ice every 24 
hours. 


In the preparation of film base or sup- 
port, cotton is thoroughly washed in cir- 
cular rotary vats with caustic soda solution 
to remove vegetable gums and other im- 
purities. After carefully drying in huge 
dryers to eliminate all moisture (Figure 2), 
it is treated with two acids, nitric and sul- 
phuric, a process known as nitration. Ni- 
‘trating centrifugals, made of perforated 
baskets rotating inside a vat, are used for 
this process. The cleansed cotton is fed 
into the basket and the acids run in until 
the cotton is immersed (Figure 3). The 
fibrous structure of the cotton is not de- 
stroyed by nitrating but the treatment makes 
it possible to dissolve the cotton later in 
a solvent. When nitrating is completed the 
acids are drawn off and the basket rotated 
at high speed for draining. Nitrated cotton 
is known as cellulose nitrate. The excess 
acid is removed by placing the nitrated cot- 
ton in centrifugal washers. After washing 
in these machines, it is placed in large tanks 
of water where it is drained and rinsed re- 
peatedly: for several weeks. Centrifugal 
wringers operated at high speed next re- 
move all the water. All these elaborate pre- 
cautions are necessary in order that the 
cotton be freed from every trace of acid. 


Washing and drying completed, the ni- 
trated cotton is ready for dissolving in the 
organic solvents. These are usually com- 
pounds such as methyl alcohol to which cer- 
tain other higher boiling liquids may be 


sufficient. 


=a 


referred to as “dope.” It is thee piped : 
large air tight tanks until ready for ce 
ing (Figure 4). To remove any undissolved 
specks and fibres, the dope is filtered und 
great pressure. It is then Wehadiee. 


solvents evaporate, the film dries, and is 
peeled off. The thin sheets of trenipateng ; 
base 2,000 feet long, 3% feet wide, and a 
proximately 5-1,000ths of an inch in thick: 
ness are wound up temporarily until ready 3 
to be coated with the emulsion. : 4 


For more pleasing presentation on he : 
screen, motion pictures are often tinted by 
bathing the flm in dye solutions which stain — 
the gelatin. To save the finisher, the time 
and trouble of this operation, Eastman posi-_ 
tive film is supplied in several different col-— 
ors of tinted bases. In this product the col- 
or is impregnated in the film base. ery 


For use in portable projectors which r. 
quire a non-inflammable film, a special safer 
base called cellulose acetate is manufacture a 
It is made in much the same way as t 4 
nitrate except that acetic anhydride is u 
instead of nitric acid for treating the cottor 
so as to render it soluble in the orga ic 


solvents. 


Preparing and Coating sig 


We now turn to the making ats the of 
sion or the light sensitive layer that | ‘ 
the photographic image. It is. made_ in 
grades, negative emulsion which is very 
sitive to light and is used in the 
and positive emulsion which is much less 
light sensitive and is used for printing the - 
pictures afterwards viewed on_ ‘the screen, 
All emulsion making is conducted i in, 
lighted with safelights which have bie e 
cially preperie for ee PHOS ; 


ee 


in size wee in WP: ee he 
are less than 1-10 as large. — 


Silver, as used in pia '6 


vices: (Figure 5). The bars are 
in nitric acid and after recrys’ 
porcelain dishes pure crystals of 
trate are obtained. (Figure 6). 


‘auIyoOeU Surzeod (348A AMO) 
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aseioys adop,, (74511 doy) p eanSy ‘fuo0zjoo Sues (az9;uaes doz) ¢ sinsyg ‘faahap u0z}09 9y} sI (79, doz) Z oansiy 


66 


gredients of the emulsion are potassium io- 
dide,» potassium bromide and gelatin. If 
these bromide and iodide salts are dissolved 
in water and to the solution thus prepared 
silver nitrate solution is added, an insoluble 
yellow salt is precipitated which is very sen- 
sitive to light, turning black after a few 
minutes exposure. 


If this solution is coated on the base, the 
film would have very little sensitiveness and 
for all practical purposes it would be worth- 
less. For this and other reasons the pre- 
cipitation must be conducted in the presence 
of some material that will avoid these dif- 
ficulties. 


The material commonly employed is gela- 
tin, a substance analagous to glue in com- 
position, and lixe glue in that it is extracted 
from the bones and hides of cattle. Photo- 
graphic gelatin is usually prepared from calf 
skin by soaking the skins in lime water, and 
subsequently extracting with hot water. The 
gelatin is dissolved in water and the bromide 
and iodide solutions carefully mixed with 
it. To this mixture heated to the correct 
temperature, is added the silver nitrate solu- 
tion. The precipitate of the sensitive silver 
salt is held in suspension throughout the 
solution by the gelatin and because of this 
it receives the term, “emulsion.” 


These actual operations are conducted in 
silver lined steam jacketed vessels provided 
with usitable agitators. Soluble salts formed 
during the reaction must be washed out of 


| ae 


the emulsion. 


Figure 8—Every photographic image is composed of coke-like masses of silver grains— i, = 
shown here under a powerful microscope. a 


This is accomplished by chil- — 
ling it to a jelly, shredding it by pressing — 
the mass through a chamber with a per-— 
forated bottom and sides, and washing the 7 
spaghetti-like strands many times with cold — 
water. The shredded emulsion is then melted — 
and coated. as 


For coating the emulsion on the hase, spe- 
cial and delicate machinery is necessary in — 
order to carefully control the thickness. The © 
film base is handled in such a way that ot 
one side comes in contact with the heated ° 
emulsion. (Figure 7). After the film F 
coated, it is carried in large loops through ~ 
chilling rooms to set and harden or become — 
“conditioned.” When thoroughly dried, it — 
is automatically cut into strips 13% inches 
wide and wound into rolls varying from 100. 
to 1,000 feet in length. 


Perforating the film is carried on in a ~ 
special department and the greatest care — 
is required to have the work done accurately, 4 
for unless the perforations are correct in — 
spacing, the film will not run smoothly in~ 
cameras, printers, or projectors and the pic- 4 
ture will be unsteady on the screen. The 
rolls of perforated film are then taken to — 
the packing room to be wrapped in selected — 
pure black paper and packed in tin cans 
which are sealed to keep the contents air 
and light tight. The cans are stamped with 


the emulsion number, the footage, and are 
then placed in strawboard containers ready — 
for shipment. . a 

To make film of the high average quality — 
demanded, 


inspection tests are necessary at — 


wegen pee 


every step in the manufacture. These in- 
clude the actual making of pictures which 
are projected to show the photographic qual- 
ity and to test the strength and wearing 
properties of the base. Thousands of feet 
of film are used up weekly in this way in 
a critical inspection of the manufactured 
product. No stock is permitted to reach 
the consumer which does not come up to 
the standard requirements. 


Research on the Chemistry of 
Emulsions 


In the manufacture of photographic emul- 
sions, the art has preceded the science. 
Great refinements have been introduced in 
manufacture on a large scale but the real 
chemical causes and the factors controlling 
the reactions have until recently remained as 
much a mystery as in the early years when 
all emulsions were coated by hand. As a 
result of a large program of intensive re- 
search that has been in progress now for 
many years in the Eastman Research Lab- 
oratory and other laboratories, some of the 
uncertainty has been removed but much ad- 
ditional work remains to be done. 


To gain a better understanding of this 
research work, something should be known 
of the actual characteristics of the emulsion. 
Ii a piece of exposed and developed motion 
picture film is examined under a high power 
microscope, the image will be found to be 
composed of minute grains or clumps of 
metallic silver, resembling tiny masses of 
coke. (Figure 8). These grains are de- 
rived from the original grains of the emul- 
sion, which under the microscope are found 
to be crystals varying in shape from spheres 
to triangular or hexagonal plates in the 
larger grains. (Figure 9). They are of all 
sizes from very small grains to quite large 
ones and the properties of the photographic 
emulsion depend largely upon the various 
sizes which are present. 


One part of this comprehensive plan of 
research has been the determination of the 
systematic relations which exist between the 
methods employed in the preparation of the 
emulsion and the photographic properties of 
the material obtained. That such relation- 
ships exist is now definitely established and 
before many years have passed a fairly com- 
plete understanding of these will have been 
arrived at. 


One phase of this investigation has been 
the direct microscopic study of the grains 
in thousands of different samples of emul- 
sions. This type of research is exceedingly 
tedious and progresses very slowly but it 
has proven one of the best lines of attack 


Figure 9—Silver Halide Grains of a 
Photographic Emulsion 


on the problem. It will not be possible to 
fully describe the method but some idea of 
its complexity may be gained from the fol- 
lowing statement. The emulsion sample is 
coated as a layer only one =tiny grain in 
thickness by a scheme requiring a high de- 
gree of skill. A minute area of this layer 
is then photographed so as to enlarge it 
10,000 diameters. The grains are next meas- 
ured, classified according to size and from 
the results of hundreds of thousands of such 
measurements, a tentative conclusion may 
be drawn. This is essentially a_ statistical 
method of attacking the problem. 

The chemistry of gelatin has also come in 
for a thorough study. That this is well worth 
while was forcibly proven by the recent dis- 
covery of a group of chemical substances 
which must be present in samples of gela- 
tin even .though in very small amounts in 
order that the gelatin be useful for making 
photographic emulsions. 

These great problems of the chemistry of 
the preparation of the sensitive materials 
are only one part of the entire problem; 
the other is the use of the photographic 
materials. The faithfulness with which the 
final print reproduces the different tone gra- 
dations of the subject under various light 
conditions is known as the problem of tone 
reproduction. It may be reasonably said 
that this problem is fully solved and a state- 
ment of the accuracy of the reproduction 
of the tone gradations. of any subject is now 
possible on any photographic material under 
any given condition of. illumination. 


Color Sensitivity of Motion Picture 
7 Films 

When a beam of white light (usually sun- 
light) is passed through a prism it spreads 
out into a multi-colored band called the 
Visible spectrum. The normal eye can dis- 
tinguish several prominent hues in this spec- 
trum, violet at one end, then blue, green, 
yellow, orange, and red.’ If this colored 
spectrum is photographed upon ordinary 
film, only the violet and blue would be com- 
pletely recorded and the green very slightly 
while the yellow and red would have scarcely 
any effect at all. A red object therefore, 
which appears relatively bright to the eye 
photographs as black whereas blue and vio- 
let objects photograph as white. The result 
is a false reproduction of almost the entire 
range of color tones. The chemist was re- 
sponsible for making photographic emul- 
sions sensitive to colors. It was found that 
on adding certain dyes called sensitizing 
dyes the sensitiveness of the emulsion to 
green and yellow was increased. Such emul- 
sions are called orthochromatic emulsions. 
Negative motion picture film is of this type 
but is relatively insensitive to red light and 
may be handled safely in darkrooms lighted 
with red safelights. It is manufactured in 
two speeds, par-speed and super-speed film; 
the latter being about twice as sensitive as 
the former. Within the past twenty years 
other sensitizing dyes have been discovered 
which on incorporation in emulsions made 
them sensitive to the entire spectrum. An 


Figure 10—Spectrum photograph showing 
sensitivity range of several emulsions. 


emulsion of this type is known as a pan- 
chromatic emulsion. (Figure 10). 
color photography has been made possible 
by the chemist’s discovery of these dye sub- 


stances and their use in the manufacture ~ 


of panchromatic film. Such pictures as 
Douglas Fairbank's “Black Pirate’ could 
never have been produced without panchro- 
matic film. 


Color Filters for Absorption.—Although 


panchromatic motion picture film is strongly | 
sensitive to red, yellow and green, it re-_ 


mains more sensitive to blue and violet espe- 
cially when photographing by daylight. To 
correct for this extra sensitiveness to the 
blue and violet, color filters are used be- 
fore the lens. These filters consist of thin 


Natural — 


Se re ee 


sheets of dyed gelatin cemented between two ‘ 


pieces of optical glass. The dyes are care- 


fully selected with reference to the por- — 


tions of the spectrum which they trans- 
mit and absorb. For example, 
filter is most commonly used with panchro- 
matic film since this filter absorbs a definite 


a yellow 


portion of the violet and blue light to which © 


the emulsion is most sensitive thereby equal- 
izing the exposure for all the colors. 


tones of the subject. 


When exposed to daylight or are lamps, — 


Thea 
result is a more accurate rendering of the 


——— 


Eastman Panchromatic Negative Film is | 


about equal in speed to Eastman Negative 
Film, regular speed. 


Negative Film. Because of 
keeping qualities and its accurate rendering 


of tone values, panchromatic film is now 


being used extensively for both portraiture 
and landscape work. 

Panchromatic film can be supersensitized 
by bathing for 1% minutes in 4 percent am- 
monia at 50 degrees F., and drying as rapidly 
as possible. When given this treatment, the 


a a, ere 


With tungsten lamps, A 
it is considerably faster than standard speed — 
its excellent — 


—_— 
x. 


film is known as hypersensitized film and is — 


about as fast as super speed negative film 
for daylight work (see Figure 10). It should 
be used as soon as possible after hypersen- 


sitizing but if necessary to store for a week — 


or so, it should be kept dry and at a tem- 


perature not higher than 50 degrees F. The © 
red and green sensitiveness of the film is — 


increased three or four times by this hyper- 


sensitizing treatment which is a great ad- 


vantage if exposures through red filters are 
to be made. 


By the use of appropriate filters and © 
treatment with certain sensitizing dye solu- — 
tions, panchromatic film finds important ap- — 
plications for making “night scenes” in the © 
daytime (Figure 11), and for making distant 
It may also be used 
for making duplicate negatives from posi- ~ 


shots through haze. 


Figure 11--“Night scenes” photographed in 
daylight on specially sensitized film. 


tives on tinted base when no other print 
is available. Colored spots and stains can 
be eliminated by duplicating in this way. 

More complete information regarding pan- 
chromatic film is given in the booklet “East- 
man Panchromatic Negative Film for Motion 
Pictures,” supplied on application to the Mo- 
tion Picture Film Department, Eastman Ko- 
dak Company, Rochester, N. Y. 


‘Chemistry in the Processing of Motion 
Picture Film 


After manufacture, motion picture film 
has little contact with chemistry until it has 
been exposed and is ready tc be processed. 
The various treatments which it then re- 
ceives such as development, rinsing, fixation, 
washing, and drying, are all chemical and 
determine in large measure the future per- 
manency of the film. Besides the action of 
the different solutions in processing the film, 
there is considerable chemistry involved in 
the actual mixing of the solutions and in the 
action of the liquids on the vessels or tanks 
used for containing them. 

Too little thought is usually given to the 
preparation of solutions used in photography. 


“NI 


We are apt to be satisfied to dump the 
cheniicals into the water, stir the bath cas- 
ually and proceed with the more important 
business of processing the film. Conversely, 
it is true, that it is unnecessary to take too 
great precautions and waste too much time 
in mixing the solutions, but more care should 
be exercised than is usually given. 

Although distilled water or rain water are 
to be preferred for mixing solutions, experi- 
ence has shown that it is only rarely that tap 
water which usually contains dissolved salts 
cannot be used. Providing the solution 1s 
filtered through a canvas cloth or allowed to 
settle before drawing off for use, very little 
trouble need be anticipated. The important 
thing is, however, to use only pure chemicals, 
dissolve each separately before adding the 
next, always mix them in the order recom- 
mended, agitate the entire volume of solution 
thoroughly as each constituent is poured in, 
and finally make up the solution to a definite 
volume with cold water. Hydrometer meas- 
urements are best avoided in mixing solu- 
tions (unless it is impossible to keep the 
chemicals dry), because it takes considerable 
time to adjust the strength of the solution. 
Hydrometer readings also vary with the tem- 
perature and no idea is conveyed as to the 
percentage strength. 

A good arrangement for mixing the solu- 
tions is to place the chemical room directly 
above the developing room. Wax impreg- 
nated wooden tanks, enamelled vats or 
smoothly glazed earthenware crocks are 
recommended as containers connected with 
chemical lead piping to convey the solutions 
to the developing and fixing tanks in the 
room below. 

Further details may be found by consult- 
ing the chapter on “Preparing Solutions” in 
the booklet, “Elementary Photographic 
Chemistry,’ published by Eastman Kodak 
Company. See also “The Development of 
Motion Picture Film’ by J. I. Crabtree, 
Trans. Soc. M. P. Eng. No. 16, p. 163 (1922). 


Developers and Development 


The purpose of a developing solution is to 
change the exposed silver salt in the emul- 
sion to metallic silver without affecting the 
unexposed silver salts. The constituent of 
the developer which accomplishes this change 
is called the reducing agent. The reducing 
agents now generally employed are elon, 
hydroquinone, pyro, and glycin. These sub- 
stances are ineffective as developing agents 
until the solutions are made alkaline, usually 
with sodium carbonate, which activates the 
reducing agent. In the presence of the oxy- 
gen of the air, however, the reducing agent 
is oxidized and the solution turns brown. A 


product somewhat like a dye is formed which 
stains the film and slows up the developing 
power of the solution. When the carbonate 
is added this rate of oxidation is increased, 
but if sodium bisulphite or sodium sulphite 
is added, the oxidation tendency is reduced 
and the solution turns brown very slowly. 
The sulphite, therefore, generally should be 
dissolved first as it acts as a preservative. 
Besides the reducing agent, the activitator, 
and the preservative, the developer contains 
a restraining agent or potassium bromide 
which assists in controlling the rate of de- 
velopment and preventing developer fog. 


The various reducing agents differ con- 
siderably in their rate of development: elon, 
for example, develops the image much 
more rapidly than hydroquinone, but on pro- 
longed development they produce similar 
images (Figure 12). Both these developing 
agents are usually added to a developer be- 
cause hydroquinone, when used alone, de- 
velops too slowly, especially at low tem- 
peratures. For negative development, when 
soft images are desired, the proportion of 
elon should preponderate, while in the case 
of a jeositive developer, when more contrast 


cL 


re 
aan. 


MIN. tein. 


are 12—Graded strips eae comparative rates of develogaaan of (E) Elon 
and (H) Hydroquinone. 


is wanted, the hyarceiaean ‘seni be in ex % 


cess of the elon. 
The difference between the 
blackness of the silver image of the lowest 


exposure and the highest exposure is a meas- 


ure of the “density contrast” of the nega- 


density a 


tive. This difference in density increases with 


time of development, 
usually occurring in the first 5 or 7 minutes 
of development. Every picture is really a 


series of varying tones and the particular 


developer used, the time and temperature of 


development of both the negative and the — 


positive print all influence the range of the 
density value of the tones. 


If development is continued too long, a 


chemical reduction of the unexposed grains 


of the emulsion takes place which is com- 
monly spoken of as “fog.” It is never ad- 
visable to develop longer than one minute 
less than the fogging point, and it is there- 
fore important to know the time required 
to produce visible fog with the type of film 


being used. Occasionally substances get in 


the developer which fog emulsions very 
rapidiy. A serious trouble of this nature was 


traced to the presence of certain bacteria — 


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or fungi which acted on the sulphite in the 
developer, changing it to sodium sulphide, 
which fogs film very badly. Both the cause 
and a method of eliminating the trouble were 
worked out in the Eastman Research La- 
boratory in connection with an extensive in- 
vestigation dealing with the classification of 
different types of chemical fog and the 
chemistry of developer solutions. 


Effect of Temperature.—The properties 
of a developer solution are affected con- 
siderably by temperature, especially if there 
is much hydroquinone present. When the 
temperature is raised, development is faster 
and with a lowering of temperature, the 
development rate is retarded. The fogging 
point also changes with temperature. In 
warm weather developers do not keep as 
well because the higher temperature in- 
creases the rate of aerial oxidation. It is 
very important in view of these facts to 
know the temperature of the solution and 
to keep it fairly constant in order to dupli- 
cate results. 


Tropical Procedures.—For handling and 
processing film under tropical conditions, a 
different technique is required. Standard 
methods have been worked out for insuring 
that the sensitiveness of the emulsion shall 
remain unimpaired and that the latent image 
be retained after exposure and before devel- 
opment. The secret of high temperature 
development is to prevent excessive. swelling 
of the gelatin. The most successful method 
of doing this is to add an anti-swelling chem- 
ical such as sodium sulphate, or sodium phos- 
phate to the developer and immerse the film 
in a hardening bath after development and 
before fixation. Such a hardening solution 
may be prepared with potassium chrome 
alum in 3 per cent. concentration, which 
works very well at temperatures from 75 to 
85 degrees Fahrenheit. When higher tem- 
perature up to 95 degrees Fahrenheit are 
encountered, about 12 per cent. sodium sul- 
phate should be added to this bath. Fixation 
can be conducted in the usual way after three 
minutes treatment of the film in this harden- 
ing solution. More complete data are given 
in a paper on “Handling Motion Picture Film 
@eaiigh Temperatures.” by J. I. Crabtree, 
feeties rans. Soc. M. P. Eng. No. 19, p. 39 
(1924), | 


Life of Developers.—The life or period 
of usefulness of a developer depends on its 
particular composition and whether it re- 
ceives continuous or intermittent use. As a 
developer is used, the solution accumulates 
reaction products which tend to retard the 
rate of development, and it is, therefore, 


necessary to develop for a longer time te 
secure a given contrast. Since reaction 
products slow down development, if a rack 
of film is allowed to remain stationary 
in, a tank) there «is: an- accumulation’ ot 
these by-products in the vicinity of the 
film which induces further retarding effects 
(Figure 13). Agitation of the rack and of 
the solution will prevent this trouble and 
give more uniform development. 


With use a developer may become ex- 
hausted in several ways: (1) By aerial oxida- 
tion; (2) by accumulation of products re- 
sulting from the decomposition of the de- 
veloping agents; and (3) by formation of 
sodium bromide and iodide from the reduc- 
tion of these silver salts in the emulsion to 
metallic silver. An old developer may there- 
fore have to be discarded because it develops 
too slowly or gives excessive stain or fog. 
When film is developed on a reel the solution 
is freely exposed to the air and if the 
developer does not contain an excess of pre- 
servative, chemical fog is produced. Ex- 
perience has shown that the addition of about 
5 per cent of old developer to a freshly mixed 
new developer will lower the tendency for 
chemical fog more than if the bromide con- 
centration is increased above the normal 
quantity added. The chemical explanation 
of this effect is probably that the oxidized 
developer acts as an anti-fogging agent. 
Work is still in progress, however, to find the 
best method of reviving used developers. 

For an extensive discussion of develop- 
ment, reference should be made to the paper, 
“The Development of Motion Picture Film 
by the. Reel and Tank’ Systems," by. J). 1. 
Crabtree; Trans. Soca MiePs, BngmeNo sale, 
De 1Oast 19227, 


Developer Troub!es.—Occasionally during 
processing, troubles arise caused by spots, 
marks, and stains appearing on the films. 
Methods of preventing, and removing many 
of these difficulties have been found and pub- 
lished, and data on others are being assembled 
and investigated. 


Stains may result from using old develop- 
ers containing an excess of oxidation prod- 
ucts, from particles of chemical matter in the 
air of the room settling on the film, or from 
undissolved solid chemicals in the developer. 
They are sometimes produced by the forma- 
tion of a scum on the surface of the develop- 
er due to insoluble oxidation products rising 
to the surface. Spots result from similar 
causes and in addition may be produced by 
bubbles of air clinging to the film on im- 
mersing in the developer or by splashes of 
oil which repel development. General stain 
may be the result of chemical fog or a 


Figure 13—Streaks Caused by the Restraining Action of Products of Development. 


coloring of the gelatin by an oxidized devel- 
oping agent such as pyro. For full discus- 
sion of stains, see paper on this subject by 
J. I. Crabtree, Amer. Ann. of Phot. 1921, 
p. 204 or Brit. J. Phot. 68, 294 (1921). 

When film is developed on racks in a tank, 
markings quite often occur at the points 
where the film passes over the top and bot- 
tom slats of the rack. These marks are 
caused by convection currents set up by the 
flowing away of chemical oxidation products 
as the development progresses. (Figure 14). 


TOP OF RACK 


“BOTTOM OF RACK _ 


Figure 14—Rack Marks produced where 


film passes over slats of rack. 


They may be very much diminished by agita- 
tion of the rack, by moving the film on the 
rack, or by the use of a special roller rack 
which permits easier movement of the film 
on the rack at intervals during the progress 
of development. (See paper on “Rack Marks 
and Air Bells,” by J. I. Crabtree and C. E. 
Ives published in the Trans. Soc. M. P. Eng. 
No. 24 p. 95 (1926). 


Chemistry of Fixation 


A fixing bath contains as the active chemi- 
cal agent sodium thiosulphate or hypo 
which dissolves the unexposed silver salts 
without affecting the silver image. A double 
salt of silver and sodium thiosulphate is 
formed which is very soluble in water and 
may be removed from the gelatin by wash- — 
ing. Hypo is seldom used as a plain solu- 
tion but usually in conjunction with a 
weakly acid salt such as sodium bisulphite 
or with an acid hardening solution. The 
standard hardener contains a preservative, 
sodium sulphite which prevents decomposi- _ 
tion of the hypo; an acid, usually acetic 
acid, to neutralize any alkali carried over in 
the film from the developer, thereby arrest- 
ing development since an acid developer will 
not reduce silver salts; and a hardening 
agent, either potassium alum or chromium 
alum. = 


It is important to mix a fixing solution 
correctly. The hardener should be prepared 
separately by dissolving the sulphite first and 
when it is completely dissolved add the acetic 
acid. After the sulphite-acid solution has 
been thoroughly mixed, add the potassium — 
alum. When the alum has dissolved make 
up to final volume with cold water and add 
the hardener solution slowly to the cold 
hypo while stirring the latter rapidly. 


There are certain criteria used in judging - 
the efficiency of a fixing bath as follows: 


ee 


Rate of Fixation—When film is immersed 
in a fixing bath, it is considered fixed when — 
it has remained in the solution twice the — 
time for the milkiness or opalescence of the 
unreduced silver salts to disappear. e254 


a ee 


*, 
, 


rate at which this takes place depends on 
the strength of the hypo (30% to 40% hypo 
fixes most rapidly), the emulsion used, that 
is whether negative or positive, the tem- 
perature of the solution (65° F. is recom- 
mended), and the degree of exhaustion of 
the solution. 


Hardening Properties——A certain mini- 
mum of alum is required to give the neces- 
sary hardening while an excess of alum may 
produce too much hardening and induce 
brittleness. Normal fixing baths are care- 
fully compounded to give a hardening of 130° 
to 170° F., determined by immersing a strip 
of the fixed and washed film in water and 
heating the water slowly until the gelatin 
flows away from the support. 


* 
ca 
= 
‘ 
. 
é 


i. Sludging Tendency.—A fixing bath may 
-_-become cloudy or precipitate a sludge in two 


different ways: (1) the hypo may break 
down giving a pale yellow sludge of sulphur 
which is the result of the temperature of 
the bath rising too high or of adding too 
much acid to the bath; and (2) the alum may 
be decomposed and a white sludge of alumi- 
num sulphite formed which is the result of 
too low acidity, the presence of excess de- 
veloper carried into the bath, or too high a 
sulphite concentration. 


Effect of Temperature.—Changes in tem- 
perature of the fixing bath affect the rate of 
fixation and the life of the solution. If a 
film requires 95 seconds to clear at 65° F., 
for example, it would take about 50 seconds 
to clear at 85° F., but it is dangerous prac- 
tice to allow the temperature of the bath to 
rise above 65° F., as the solution is apt to 
precipitate sulphur. Under tropicai condi- 
tions where high temperatures and high 
humidities prevail, it is obviously often im- 
possible to keep the temperature within this 
limit, and the fixing bath must usually be re- 
placed oftener. A different technique must 
be used for tropical processing as mentioned 
previously under the subject of development. 


Life of Fixing Solutions.—As a fixing 
bath is being used, the hypo becomes ex- 
hausted as a result of performing useful 
work in fixing out the emulsion. When the 
time for clearing negative film exceeds a 
certain point, say ten minutes, the bath 
should be discarded. The acidity of the 
bath is being reduced by the developer car- 
ried in although at first this tends to favor 
a longer sulphurization life. With continued 
use, however, the solution finally reaches a 


point where a sludge: of aluminum sulphite 
is precipitated rendering the bath useless. 
On the other hand, the hardening properties 
of a bath usually increase slightly during the 
first stages of use after which they fall off 
rapidly until the bath is revived. 


Revival of Fixing Baths.—Since the acidity 
of a bath and its hardening properties are 
depleted before the hypo is used up, it is 
general practice to revive the bath at in- 
tervals by the addition of a definite quantity 
of acetic acid. When the acidity is lowered 
to two-thirds of the original value, enough 
acid should be added to restore the initial 
concentration. A fairly satisfactory method 
is to add acid after a certain footage of film 
has been passed through the solution. It the 
bath has formed a sludge before it was re- 
vived, the solution should be discarded. 


Recovery of Silver—An exhausted fixing 
bath contains dissolved silver salts and vari- 
ous methods have been tried to profitably 
recover the silver. The principal methods 
used are: (a) the sulphide method wherein 
sodium sulphide is added to the used bath 
and the silver is thrown down as a sludge 
of silver sulphide; (b) the hydrosulphite 
method in which the addition of sodium 
hvdrosulphite precipitates a mixture ot me- 
tallic silver and silver sulphide depending on 
the conditions of precipitation; (c) the zinc 
method which uses zinc in various forms, 
such as sheet, granulated, and dust; the sil- 
ver being precipitated as metallic. silver; 
(d) electrolytic methods which include the 
use of metallic units and the actual applica- 
tion of an electrical current. For recovery 
of large quantities of fixing bath, the sul- 
phide method is most efficient; for medium 
quantities, the zinc dust method is satisfac- 
tory, and for small quantities, the use of an 
electrolytic recovery unit offers a simple 
and economical procedure. 


Fixing Bath Troubles—When the carbo- 
nate in the developer is neutralized by the 
acid in the fixing bath, carbon dioxide gas 
is formed which produces blisters which ap- 
pear on the film as tiny craters providing 
the gelatin is too soft to withstand the dis- 
ruptive action of the gas. If the bath has 
good hardening properties and the film 1s 
agitated on first immersion no trouble from 
blisters need be anticipated. If the fixing 
bath does not contain acid or if it is old and 
exhausted and contains an excess of dis- 
solved silver, a chemical fog called dichroic 
fog is sometimes produced on the film. In 
reflected light, film fogged in this way looks 
yellowish green and by transmitted light it 
appears reddish pink. Dichroic fog never 


occurs in a fresh acid fixing bath or if the 
film is rinsed before fixing and the tempera- 
ture of the bath is kept at 65° to 70° F. 
When a partially exhausted fixing bath is 
allowed to stand several days without use, 
the hydrogen sulphide gas present in small 
quantities in the. air reacts with the silver 
thiosulphate in the bath and forms a me- 
tallic-appearing scum on the surface of the 
solution. The scum consists of silver sul- 
phide and should be removed by drawing the 
edge of a sheet of blotting paper across the 
surface of the bath. Trouble from sludging 
and precipitation has been discussed pre- 
viously. Several different stains such as 
white aluminum sulphite stain, sulphur stains 
and yellow silver stains are occasionally pro- 
duced. More complete discussion of fixing 
troubles is given in a paper on “Stains on 
Negatives and Prints,” by J. I, Crabtree, 
Brit. J. Phot. 68, 294 (1921). 


Chemistry of Washing 


One might naturally think that there 
is little chemistry associated with washing 
film, and this is true so far as actual chemi- 
cal changes are concerned, but it is a dis- 
tinctly physico-chemical problem to deter- 
mine the conditions that will ensure com- 
plete removal of hypo and other fixing bath 
components and oxidized products from the 
film. The nature of the water supply is not 
of vital importance, although if dirty water 
or sea water have to be used, the film should 
be subsequently given a thorough washing 
in fresh water. 


The problem of washing film involves two 
principal operations: (1) separation of the 
chemical substances from the film, and (2) 
removal of these substances from the water 
in the vicinity of the film. Obviously the 
second operation must proceed equally as 
rapidly as the first, or the film would still 
retain some chemicals when taken from the 
wash water and stains would appear later. 
The first operation is really a problem of 
diffusion since the chemicals are held in 
the swollen gelatin layer and must find their 
way out to the surface. It has been found 
that as washing progresses under favorable 
conditions, the hypo content of the film is 
halved for each equal time interval. This 
“half period” value can be determined for 
each type of film. Most thorough washing re- 
sults if the water is violently agitated at the 
point of contact with the film. Practically 
the ideal washing stream can only be real- 
ized by using a spray or an excessively large 
flow of water. Tanks should be as small as 
is consistent with the film output because 
the smaller the volume the more rapidly is 


12 


the stale water removed. To remove sur- 
face hypo, a few seconds rinse in a separate 


tank, or spraying with a coarse atomizer 


previous to the main washing is recom- 


If it can be arranged, the cascade 
In this method, film 


mended. 
system is excellent. 


is transferred successively through about 


five baths in which the water is circulating 
in a counter direction. In continuous tube 
processing machines, the water should be 
directed into the top of the last. tube, flow 
from the bottom into the top of the next 
tube, and so on. A rinsing loop previous to 
the large washing tubes should be installed. 


Compressed air admitted at the bottom of 4 


tank or tube provides the most economical 


and efficient method of agitation. 


Washing Troubles.—If washing is incom- 
plete as pointed out under the subject of 
fixation, trouble from stains and spots is 
often experienced. Sometimes these diffi- 
culties do not appear until several months 
or a year or two later, but the film is usu- 
ally seriously if not permanently damaged. 
Several theories have been advanced to ex- 
plain this deterioration, but it is most prob- 
able that the sulphur liberated from small 
traces of retained hypo in conjunction with 
bacterial action are the chief causes of the 
fading that take place through the forma- 
tion of silver sulphide. Most trouble is ex- 
perienced with film that has been poorly 


processed and stored in hot damp climates. 


If the film is 
troubles will arise from having to wash in 
water whose temperature is 90° to 100° F. 
When film is thoroughly fixed and washed 
and subsequently stored at high tempera- 
tures, it rapidly becomes brittle and in a 
few years is completely destroyed by its own 
decomposition products. 
ant, therefore, to store film when practicable 
at temperatures of 50° or 40° F., when the 
rate of decomposition is negligible providing, 


It is very import-— 


properly hardened no © 


of course, that the proper care has been 


given the film during the processing opera- 
tions. The motion picture industry is com- 
paratively young, but there are samples of 


film in very good condition which were 


processed over thirty years ago. For more 
detailed information on the subject of wash- 
ing motion picture film, see the article by 
K. C. D. Hickman, Trans. Soc. M. P. Eng. 
No. 23, p. 62 (1926). 


When moisture comes in contact with dry 
film previous to or after exposure, or is de- 
posited as the result of humid atmospheric 
conditions, or is left on the film previous — 
to or during drying, certain characteristic 


Figure 15—“Moisture Marks.” 


markings are produced. The most common 
example of these is a tiny white spot. 
(Figure 15). Various other types have been 
classified and several explanations advanced 
as to the causes and means of preventing 
such markings in a paper on the subject 
published in the Trans. Soc. M. P. Eng. No. 
17, p. 29 (1923). 


Corrosion and Its Relation to Con- 
struction Materials for Photo- 
graphic Apparatus 


In selecting a material for the construction 
of photographic processing apparatus, it is 
important to know the probable effect, if 
any, of both the material on the solution 
and the chemical action of the solution 
On the material. A metal like tin, for 
example, which is entirely satisfactory for 
pipe lines carrying distilled water, is on the 


_ other hand very unsuitable for use for con- 


structing developer tanks as it reacts with 
the solution giving very bad fog. An inves- 
tigation of this- subject was made several 
years ago and the results published in a 
series of papers to which reference should 
be made for complete information. (See 
Amer. Phot. 18, i148, (1924) ). A few of the 
conclusions may be of interest. Tin, copper, 
_ or alloys containing these metals should not 
come in contact with developers as serious 
trouble from fog will be experienced. Sold- 
ered joints in metal tanks are to be avoided. 


If metals must be used for apparatus to 


contain fixing baths, nickel, lead and monel 
are the only ones recommended, and these 


- should be electro-welded or soldered from 


a the outside except in the case of lead which 
_ should be burned. Aluminum, zinc, or gal- 
 vanized iron should not be used with either 


] 


- 


developers or fixing baths as these metals 
react with the solutions forming precipitates 
which deposit on the film and stain the 
gelatin. 


Single metals or alloys are to be preferred 
to plated metals because when surface plat- 
ing becomes worn or chipped they corrode 
very rapidly. Porous earthenware, fibrous 
materials, and rubber compositions should be 
avoided since the solutions crystallize out in 
the pores and subsequently disintegrate the 
material. Lacquered trays and japanned 
tanks are not suitable for containing strongly 
alkaline developers or acid fixing baths. 
Specific recommendations relative to the 
most suitable materials for constructing 
small apparatus, trays, tanks, tubes, troughs, 
piping, pumps, faucets, etc., are given in a 
paper on that subject which may be obtained 
on application to the Service Dept. Eastman 
Kodak Co., Rochester, N. Y. 


Chemistry in the After Treatment of 
Motion Picture Film 


Reduction—Film is occasionally over- 
exposed in the camera and although this can 
be partly compensated for in the printing, it 
is customary, to treat the film with solutions 
which chemically remove some of the image. 
This process is photographically termed “re- 
duction” although it is not a chemical reduc- 
tion but rather an oxidization since that por- 
tion of the image which is removed has been 
oxidized. 


There are three general types of photo- 
graphic reducers: 


(1) Cutting reducers which remove the sil- 
ver nearly equally from all parts of the 
image; (2) Flattening reducers which attack 
the heavy deposits more than the lighter 
areas; and (3) True scale -reducers. that 
attack both highlights and shadows propor- 
tionately. Reducers of the first type are; 
(a) Farmer’s reducer consisting of potas- 
sium ferricvanide and hypo; and (b) the 
permanganate reducer which is a slightly 
acid solution of potassium permanganate. 
With Farmer’s reducer, the silver is con- 
verted to silver ferrocyanide which the hypo 
dissolves. The permanganate reducer ox:d- 
izes the silver to silver sulphate which is 
sufficiently soluble in water to be dissolved. 
The net effect with either reducer is that 
the shadows of the negative are proportion- 
ately attacked the most, since they have 
less available silver to tose than the rest of 
image. 

When a negative image has excessive con- 
trast caused by overlighting or over devel- 
opment, a reducer of the second type 1s 


necessary. An acidified solution of ammon- 
1um persulphate is the only solution known 
to act on the heavy silver deposits more than 
on the lighted ones. A fairly satisfactory 
explanation of the chemistry of this reaction 
has been reached and it is now known that 
silver sulphate is formed which dissolves in 
the solution. =. 


When it is advantageous to reduce the 
printing time on a good negative, a true 
scale or proportionate reducer (type three) 
is sometimes employed. This solution con- 
sists of a compounded mixture of the per- 
mangate and the persulphate reducers and 
it weakens the image in direct proportion 
to the amount of silver deposit present. 


Intensification.—Although several methods 
have been worked out for chemically reduc- 
ing the photographic image, it is quite an- 
other problem to add to or build up an 
image. As the saying goes, “what isn’t there 
is hard to put there.” A few solutions have 
been found, however, with which an under- 
exposed or an underdeveloped image may 
be improved. 


Usually intensification is performed by de- 
positing a silver, mercury, or chromium com- 
pound upon the image. The most common 
mercury intensifier is Monchoven’s solu- 
tion which uses a solution of mercury bich- 
loride and potassium bromide for the bleach; 
this reacts with the silver forming a mixture 
of mercurous chloride and silver chloride. 
The image may be re-developed in several 
ways, with 10% sulphite solution, with an 
elon-hydroquinone developer, with 10% 
ammonia, and with a solution of potassium 
cyanide and silver nitrate, each solution giv- 
ing proportionately greater intensification. 


A chromium intensifier consists of an acidi- 
fied solution of potassium bichromate which 
is used for the bleach. After a thorough 
washing, the image is redeveloped in a regu- 
lar developer such as elon-hydroquinone. 
The chemistry of this intensifier is not very 
well understood but its use has found in- 
creasing favor owing to the ease and cer- 
tainty of its operation and the permanency 
of the intensified image. 


Tinting and Toning.—In order to obtain 
more pleasing effects on the screen, motion 
picture film is often colored by treatment 
with various chemical solutions. Tinting is 
accomplished by evenly staining the gelatin 
emulsion or the support by means of slightly 
acidified dye solutions. It is rarely neces- 


sary to tint the emulsion since positive film 
on tinted support or base has been supplied 
for several years in nine different colors by 
the Eastman Kodak Company. This tinted 
positive film is printed and processed in the 
ordinary way. The use of tinted film thus 
eliminates several extra operations which are 
both expensive and troublesome. 

Toning consists in changing the original 
silver image to a colored inorgane salt of 
silver or to a dye image. 
commonly used methods of inorganic toning. 
In the first method known as sulphide toning 
the film is bleached in a ferricyanide-bromide 


bleach and subsequently treated with a weak 


solution of sodium sulphide which yields a 
final brown image composed of silver sul- 
phide. Very beautiful blue tones are ob- 
tained by the use of a solution containing 
potassium ferricyanide and ferric alum in the 
presence of an alkaline salt of oxalic acid, 
a mineral acid and certain other salts. The 
silver image is thereby converted to a mix- 
ture of silver ferrocyanide and iron (ferric) 
ferrocyanide which forms the blue-toned 
image. Tones ranging from chocolate to red- 
dish brown may be produced in a somewhat 
analogous way as the iron tones by the use 
of a bath containing uranium ferricyanide, 
the final image consisting of silver and uran- 
ium ferrocyanide. 


lf a silver image is converted more or 
less to a silver ferrocyanide image and the 
film immersed in a basic dye solution a mor- 
danted dye image is produced. What hap- 
pens is that the dye will attach or mordant 
itself to the silver ferrocyanide whereas it 
will not stick to the silver alone. This chemi- 
cal reaction provides a method of obtaining 
a wide range of tones which may be still 
further extended by double toning. Basic 
dyes are the most suitable for use since they 
do not readily dye gelatin. Other effects 
may be produced by combining tinting and 
toning. For detailed information see the 


booklet, “Tinting and Toning Eastman Posi- 


tive Motion Picture Film,’ published by 


Eastman Kodak Co. 


Renovating Motion Picture Film.—Aiter 
film has been projected and handled several 
times, it accumulates a certain amount of 
grease and dirt which detract from its pro- 
jection value. If this is permitted to con- 


- tinue, the film may be badly damaged by 


scratching from grit. It is customary to 
renovate the film by treating it with solu- 
tions which will dissolve the grease and 
loosen the dirt. Gasoline, benzine, toluene, 
and xylene may be used for cleaning film 


but because of the inflammability of these — 


chemicals, commercially pure carbon tetra- 


chloride is preferable. All cleaning chemi-_ 


There are three 


mh | 


> a 


SA of phe ie eid ‘) Me 2 eee 


a Rie Behe eS 6 


— Fa 


oe ee Be Penne 


cals or solvents must be used with discre- 
tion, however, and the liquid allowed to 
completely evaporate before the film is 
rewound, or the image may be subsequently 
attacked. Traces of sulphur chloride pres- 
ent in impure samples of tetrachloride prob- 
ably cause fading due to deposition of sul- 
phur which combines with the image form- 
ing silver sulphide. If pure tetrachloride 
is used and the film wound spirally on a 
drum, and the solvent applied with a soft 
cloth or velvet, the solvent will have suffi- 
cient time to evaporate before rewinding the 
film. Another non-inflammable solvent which 
does not fade the film is tetrachlor-ethylene. 
Similar precautions for cleaning on a large 
drum should be used. There are machines 
on the market in which the film passes over 
several moist felt pads saturated with sol- 
vents and then over a series of polishing 
wheels made with small pieces of velvet 
fastened around the periphery of the wheels. 
The polishing wheels rotate very rapidly and 
ensure thorough drying and polishing of the 
film before rewinding. 


It is now usual practice to apply a narrow 
line of melted wax to new or first run prints 
along the center of the perforation area 
which provides against the liability of strain 
in first projection. Similarly when film is 
renovated it should always be rewaxed as the 
cleaning chemicals remove all or nearly all 
the wax. 


Splicing and Varnishing Film.—Splicing of 
film is essentially a chemical problem since 
the film cement must possess certain prop- 
erties, such as good adhesiveness, fairly 
rapid evaporation on drying, and have no 
corrosive action on the film support. When 
film has been projected many times it 
sometimes acquires scratches which fill up 
with dirt and grease and show up plainly on 
projection. Cleaning the film removes the 
dirt from the scratches but as soon as the 
film is put into use again the tiny groves 
O\ap as much as before. To prevent this, 
varnishes have been compounded for treat- 
faethe film. Such varnishes have to be 
made very carefully as they must possess the 
saine refractive index as film base or in 
other words the varnish layer must not 
change the direction of the light rays when 
‘the film is projected. Furthermore these 
varnishes must give a hard, non-abrasive 
surface when coated very thinly on the film 
and must not attack the support, the gelatin 
or the image. 


Chemistry and Color Motion Pictures 


A field which is demanding more attention 
yearly 1s that of natural color motion pic- 
tures. This problem is both an optical as 
well as chemical one; optically it demands 
unusual refinements in the design of lens 
systems and chemically it imposes a dif- 
ficult problem in processing and in final dye- 
ing of the films. There are two general 
classes of natural color motion pictures: 
those produced by additive and those by 
subtractive methods. These are further sub- 
divided according as they use three color or 
two color ranges in color reproduction. In 
the additive process, several distinct color 
records are taken and projected separately 
and are either superimposed or shown in rapid 
succession, the colors being added to or 
built 


up on the screen. These processes 
usually require complicated and expensive 
apparatus. Whereas, in the _ subtractive 


method which has found most public favor 
the color records are taken separately but 
are finally incorporated on a single film and 
projected in the same way as standard pic- 
tures. 

In the foregoing description of the value 
of chemistry in the motion picture industry, 
it has not been possible in view of the nature 
of this article and the diversity of the sub- 
ject matter to discuss in much detail the 
actual chemistry involved. It is hoped that 
some idea may have been gained, however, 
of the importance of chemistry in every 
phase of the industry from the assembling of 
the raw material for manufacture to the final 
projection of the film. 


Additional References 


“The Fundamentals of Photography,” by 
C. E. K. Mees, published by Eastman Kodak 
Co. 

“Significant Progress in Research on 
Photography,” by C. E. K. Mees, Annals of 
Amer. Acad. Polit. Sci., 69, 10 (1925). 

“Application of Microscopy to the Photo- 
tographic Industry,” by A. P. H. Trivelli and 
Re P, Loveland, J. Rov; -Micro, Soc., 1925, -p: 
293. 

“The Home of Film,” published by East- 
man Kodak Co. 

“Moving Pictures,’ F. A. Talbot, Lippin- 
Cote bub. Co, Phila... Paz 1923. 

“Handbook of Projection,” F. H. Richard- 
son, Chalmers Pub, Co., New York, N. Y. 
“Chemistry in Industry,’ Chapt. 18. 

Chemical Foundation, Inc., 1924. 


The 


